Hermetic packaging, which provides tightly sealed cavities, has been used to protect many MEMS (micorelectromechnaical systems), such as optical, RF (radio frequency) and sensor devices, against moisture and other corrosive gases from seeping in, or to keep under controlled atmosphere. Specific examples include DLP™, bolometer, accelerometers and gyroscope. Wafer-level packaging offers advantages for packaging of cavities brings the cost advantage of simultaneously sealing an entire wafer of cavities. This eliminates the manufacturing inefficiencies and the costs of individual “pump down and pinch off” for archaic metal or ceramic packages. These potential cavity package advantages have sparked many development efforts for wafer-scale hermetic cavity packaging. The earliest cavity wafer-level packaging to be produced in large quantities were for protecting MEMS devices with moving surface elements. Millions of automotive airbag systems are today controlled by MEMS accelerometers residing in hermetic cavity wafer-level packages. More recently, cavity non-hermetic wafer-level packaging support high-volume consumer applications, such as digital cameras. Controlled-atmosphere hermetic cavity wafer-level packaging are currently being offered for MEMS RF switches. Further developments aim at size, weight and cost reductions for limited-lifetime products, or at economically meeting the more stringent requirements of high-performance, long-lifetime MEMS, optical devices and sensors.
Since cavity wafer-level packaging by their nature are generally precluded from adding layers over the active devices on the wafer surface, cavity packages are created either by bonding a second wafer with pre-formed cavities over the device wafer (wafer stacking) or by dicing the second wafer and bonding the individual cavity chips onto the device wafer (chip-on-wafer).
The present invention relates to manufacturing hermetic packaging cavities at the wafer level by wafer bonding and forming enclosures that is impervious to moisture or ambient gas. One approach to fabricating a wafer-level cavity package is to use epoxy to bond a cap wafer with a stenciled wafer for forming open cavities on one side first. Then this wafer is likewise bonded and sealed to the substrate wafer that contains MEMS devices (such as DLP™, accelerometers), thereby sealing numerous MEMS devices on the substrate wafer in enclosure cavities. This approach is very simple and cost effective. However, because of the permeability and possible out-gassing of the epoxy seals, the package is classified as non-hermetic. Another approach is to enclose the MEMS devices in deposited film using a sacrificial layer as temporary support, which is subsequently removed by etching through small holes in the deposited films, which is in turn sealed with deposited film. This approach is only suitable for very small devices because the films are much thinner than the cap wafers made from bulk material.